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Halo velocity bias - a first order systematic error in peculiar velocity cosmology ZHENG Yi ( ) Zhang, Zheng & Jing, 2014, arXiv: 1405.7125, PRD accepted Zheng, Zhang & Jing, 2014a, arXiv: 1409.6809, PRD accepted Zheng, Zhang & Jing, 2014b, arXiv: 1410.1256, PRD submitted 1YongPyong-High1 2015 (Cosmological group at KASI) http://cosmology.kasi.re.kr/ Sampling artifact Velocity bias Important slides

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YongPyong-High1 20152 Cosmological observations: galaxy velocity field (power spectrum) Cosmological observations: dark matter velocity field (power spectrum)

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3 Peculiar velocity: a window to the dark universe Matter distribution in our universe is inhomogeneous Gravitational attraction arising from inhomogeneity perturbs galaxies and causes deviation from the Hubble flow v r v r peculiar velocity v=Hr ZPJs PPT YongPyong-High1 2015

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4 Peculiar velocity: unique probe of cosmology At scales larger than galaxy clusters, directly probes gravity ( t-t component: In linear regime, honest tracer of matter distribution Necessary for the complete phase-space description of the universe ZPJs PPT YongPyong-High1 2015

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Challenges However, accurate velocity measurement? In particular at cosmological distance, e.g. z~1? Conventional method: subtract the Hubble flow with distance indicators (FP, TF). E.g. SFI++, 6dF (e.g. Johnson et al. 1404.3799) Statistical errors blow up with redshift. Only applicable at z